1. Field of the Invention
The present invention relates to a surface treatment process for mechanical parts subject to wear and corrosion. The invention relates more particularly to a surface treatment process for mechanical parts subject to wear and corrosion which confers on said parts a high resistance to wear and corrosion and a roughness propitious to lubrication. To be even more precise, the invention relates to a surface treatment process for mechanical parts whose lubrication must be precisely controlled and whose roughness must consequently be controlled within a narrow range.
2. Description of the Prior Art
The thickness of the oil film on the surface of a part depends greatly on the roughness of its surface, as is well known in the art: a part that is perfectly polished may not be wetted by the oil whereas, conversely, a very rough part will be covered with a film whose thickness is less than the height of the microreliefs, resulting in a high risk of binding.
Parts that can advantageously be treated in accordance with the present invention include piston rods and internal combustion engine valves, for example. With regard to a piston rod, the thickness of the oil film on its surface must be perfectly controlled; if it is too thin, the rod-seal contact is no longer lubricated and wear occurs; if it is too thick, the resulting leakage of lubricant degrades performance. With regard to an internal combustion engine valve, the oil film fulfills lubrication and dynamic sealing functions in the area of contact between the valve stem and the valve guide; a part that is too highly polished will produce a thin oil film and lubrication will be random, whereas high roughness will lead to high oil consumption and loss of engine efficiency.
Many solutions are available to the skilled person when faced with a member that must resist wear and corrosion. Thus it is standard practice to use thick deposits of xe2x80x9chard chromiumxe2x80x9d with microcracks. These deposits have drawbacks, however. From the technical point of view, the presence of an interface between the steel and the chromium may give rise to dramatic scaling in the intended functions; moreover, in the case of parts that operate intermittently, such as some piston-and-cylinder devices, there is a risk of elimination of the residual film of lubricant by inclement weather and therefore of corrosion. From the economic point of view, the above process necessitates deposition followed by machining, which makes it a costly solution. Finally, from the environmental point of view, chromium-plating is still very widely practiced using baths containing chromium VI, which is a major pollutant.
Another solution that is widely used consists in nitriding the parts and then oxidizing them; these two operations are often followed by a step of impregnating the surface pores with a product further improving corrosion resistance. The above operations are carried out in succession, either in a salt bath, as disclosed in French patent FR-A-2 672 059 or U.S. Pat. No. 5,346,560, for example, or in a gaseous atmosphere, as disclosed in European patent 0217420, for example.
The combined operation of nitriding and oxidation generally imparts a very high resistance to wear and corrosion, but systematically increases the surface roughness of the part, to a degree that is incompatible with what is required for the applications to which the invention relates.
This increase in roughness leads the skilled person to add to the above processes one or more phases of more or less extensive polishing, yielding sequences such as nitriding-oxidation-polishing or even nitriding-oxidation-polishing-oxidation. Processes of this kind fulfill the lubrication function effectively, but are difficult to apply industrially because they require a combination of different technologies (thermochemical and mechanical), which makes them both very costly and of limited application; it is difficult to control the roughness of a part of complex shape by polishing.
Surprisingly, we have shown that it is possible to obtain high wear and corrosion resistance and a roughness propitious to lubrication by carrying out the nitriding and oxidation operations in particular baths.
The objects defined above are met by the present invention, which provides a surface treatment process for mechanical parts, for conferring on said parts a high resistance to wear and corrosion and a roughness propitious to lubrication, in which process nitriding of said part is followed consecutively by oxidation of said part, said nitriding is applied by immersing said part in a molten salt nitriding bath free of sulfur-containing species at a temperature from approximately 500xc2x0 C. to approximately 700xc2x0 C., and said oxidation is carried out in an oxidizing aqueous solution at a temperature less than approximately 200xc2x0 C.
To conform to the invention, the process must also conform to a consecutive association of nitriding and oxidation, which two operations are carried out in the liquid phase under the conditions specified above.
However, it is not a question of consecutive association of a particular nitriding process and a particular oxidation process, but rather of an inseparable combination of nitriding and oxidation processes because, in the process according to the invention, there is a very high level of interaction between them.
The two steps of the process, namely the nitriding step and the oxidation step, must conform to the following conditions:
(1) The first step (the nitriding operation) must be executed in a molten bath free of sulfur-containing species.
The temperature of the bath is from approximately 500xc2x0 C. to approximately 700xc2x0 C., for example from approximately 590xc2x0 C. to approximately 650xc2x0 C.
The bath advantageously includes alkaline carbonates and cyanates and has the following composition:
Li+=0.2-10 wt %
Na+=10-30 wt %
K+=10-30 wt %
CO32xe2x88x92=25-45 wt %
CNOxe2x88x92=10-40 wt %
CNxe2x88x92 less than 0.5 wt %
For example, the molten salt nitriding bath contains the following ions:
Li+=2.8-4.2 wt %
Na+=16.0-19.0 wt %
K+=20.0-23.0 wt %
CO32xe2x88x92=38.0-43.0 wt %
CNOxe2x88x92=12.0-17.0 wt %
with a quantity of CNxe2x88x92 ions of not more than 0.5 wt %.
Agitation by compressed air is advantageously provided.
The time of immersion of the parts is advantageously at least approximately 10 minutes; it can be extended up to several hours, depending on what is required. The time of immersion of the parts is usually from about 30 minutes to about 60 minutes.
(2) The second step (the oxidation operation), after nitriding, must be carried out at a temperature less than approximately 200xc2x0 C. The temperature of the oxidation bath is preferably from approximately 110xc2x0 C. to approximately 160xc2x0 C. The temperature of the oxidation bath is even more preferably from approximately 125xc2x0 C. to approximately 135xc2x0 C.
The composition of the bath is advantageously as follows:
OHxe2x88x92=10.0-22 wt %
NO3xe2x88x92=1.8-11.8 wt %
NO2xe2x88x92=0-5.3 wt %
S2O32xe2x88x92=0.1-1.9 wt %
Clxe2x88x92=0-1.0 wt %
Naxe2x88x92=1.0-38 wt %
For example, the oxidizing aqueous solution contains the following ions:
OHxe2x88x92=17-18.5 wt %
NO3xe2x88x92=4.0-5.5 wt %
NO2xe2x88x92=1.0-2.5 wt %
Clxe2x88x92=0.25-0.35 wt %
Na+=25-29 wt %
For example, the oxidizing aqueous solution further contains 0.6 to 1.0 wt % of thiosulfate ions S2O32xe2x88x92.
The time of immersion of the parts in the oxidation bath is advantageously from approximately 5 minutes to approximately 45 minutes.
It is noteworthy that, after being nitrided and then oxidized in accordance with the invention, the parts treated can then undergo an impregnation operation as effectively as in the prior art. Although the final roughness is much lower, the affinity of the layer for impregnation products is at least as high. This surprising fact has as yet to be explained scientifically.
The invention also provides a part treated by the above process, in which said process has caused surface modifications. A part according to the invention is characterized in that its roughness Ra has a value less than approximately 0.5 xcexcm and in that its surface is free of xe2x80x9ctablesxe2x80x9d.
The invention is described next in more detail by means of the following non-limiting examples.